The long-range goal of this research is to gain as complete understanding as is possible of the relationship between the structure and catalytic activity of the enzyme fomyltetrahydrofolate synthetase. This goal depends on obtaining information concerning: 1. the interaction sites between subunits in the active tetrameric form, 2. the relationship between the quaternary structure, the substrate binding sites, and the catalytic center, 3. the role of monovalent cations in the structure and catalysis of the proteins, 4. the conformational transitions produced in the protein by substrates, and 5. the chemical mechanism of the reaction. The aims of research for the proposed period of support are: 1. To investigate in more detail the self-assembly of monomers of formyltetrahydrofolate synthetase in the presence of K ion. We shall study the effects of pH, organic solvents, anions, pressure and amino acid modifications. 2. To examine conformational changes produced in the monomer by ATP and changes in temperature and in the tetramer by tetrahydrofolate. 3. To study substrate and cation binding using NMR measurements. 4. To gain more information concerning the mechanism of the reaction by studying aspects of the reaction which produces ATP formation from carbamyl phosphate. 5. To initiate studies of the mechanism and the role of monovalent cations in the formyltetrahydrofolate synthetase component of the triple-headed eucaryotic enzyme.